Enhanced thermoelectric performance of Hafnium free n-type ZrNiSn half-Heusler alloys by isoelectronic Si substitution

Abstract

In this work, cheap element Si is added into Hafnium-free ZrNiSn half-Heusler (HH) compound to reduce its lattice thermal conductivity. We report a comprehensive analysis of the isoelectronic Si substitution on the electrical and thermal conductivity for ZrNiSn half-Heusler alloys. The first-principles calculations show that Si doping has a negligible effect on the electronic structure of ZrNiSn1-xSix HH materials. The band gap of 0.48 eV for ZrNiSn sample is compared to that of 0.49 eV for Zr27Ni27Sn26Si. The transmission electron microscopy (TEM) images display the nano structure in the ZrNiSn0.98Si0.02 sample, which strongly affects the lattice thermal conductivity. The contribution of point defect scattering by Si substitution on the lattice thermal conductivity is quantitatively analyzed by using Debye-Callaway model, suggesting that the strain fluctuation is the main factor to reduce lattice thermal conductivity. Finally, the maximum zT value reaches 0.78 at 873 K for the ZrNiSn0.98Si0.02 sample.